Thermal trip device with pickup coil on an adjustable armature



May 8, 1956 K ETAL 2,744,979

THERMAL TRIP DEVICE WITH PICKUP COIL ON AN ADJUSTABLE ARMATURE 4 Sheets-Sheet 2 Filed Sept. 22, 1954 m/z/wkorio qgmm S. 93/0MAXQ 5. Some/m2 May 8, 1956 T. G. BANK ETAL 2,744,979

THERMAL TRIP DEVICE WITH PICKUP COIL ON AN ADJUSTABLE ARMATURE Filed Sept. 22, 1954 4 Sheets-Sheet 3 May 8, 1956 T, BANK ETAL 2,744,979

THERMAL TRIP DEVICE WITH PICKUP United States Patent THERMAL TRIP DEVICE WITH PICKUP COIL ON AN ADJUSTABLE ARMATURE Thor G. Bank, Milwaukee, and Willard S. Lorenz, Occnomowoc, Wis., assignors to Allis-Chalmers Manufacturing Company, West Allis, Wis.

Application September 22, 1954, Serial No. 457,744 6 Claims. (Cl. 200-88) This invention relates generally to a circuit breaker for interrupting an electrical circuit, and more particularly to a new and improved thermal trip device which is combined with a relay for eitecting the tripping of the circuit breaker.

More specifically, this invention is directed to a thermal trip device having the energizing pickup coil on an adjustable armature of the relay. The relay, utilizing the invention, will substantially instantaneously effect the controlling operation in the circuit breaker in which the relay is utilized. The operation will respond to excessive or severe abnormal current and will, in addition, effect similar controlling operation upon the recurrence of abnormal current existing for a predetermined time.

- In accordance with the invention claimed, the new and improved relay when used in its alternating current circuit breaker, includes a core and primary coil electrically connected with the circuit breaker. A movable first armature, a stationary second armature and an instantaneous trip third armature are all positioned in the magnetic circuit'of the core. A secondary pickup coil is mounted adjacent and inductively coupled with the second armature.

A thermal trip device includes a shaft and a pivoted latch. The latch engages and holds the first armature against pickup in its nontrip position. The thermal trip device further includes a bimetallic element which is mounted adjacent the shaft and the latch. One of the extremities of the bimetallic element is firmly afiixed to the shaft and the other of its extremities is firmly afiixed to the latch. A heater is placed adjacent the bimetallic element. The heater is electrically connected with the pickup coil which is mounted on the stationary second armature. The heater provides the bimetallic element with the heat necessary to cause operation of the bimetallic element.

The stationary second armature has an adjustable air gap between itself and the core for varying the amount of induced electromotive force in the pickup coil. The first armature actuates means for tripping the circuit breaker and operates in response to the bimetallic element operation which releases the latch and permits the first armature to pick up.

An object of the present invention is to provide a new and improved circuit breaker relay utilizing a thermal trip device which when associated with an electric circuit will substantially instantaneously effect a controlling operation upon the occurrence of a moderate abnormal circuit condition obtained for a predetermined time.

Another object of the present invention is to provide a new and improved thermal trip device which as part of a circuit breaker relay can be readily and accurately adjusted to etfect a controlling operation at a variety of predetermined points within a given range of moderate abnormal circuit conditions.

Other objects and advantages than those set forth, will be apparent from the following description when read in connection with the accompanying drawings, in which:

Fig. l is a side view in elevation of the circuit breaker embodying the invention, portions of the breaker are broken away to show greater details;

Fig. 2 is an enlarged plan view showing the thermal trip device in greater detail;

Fig. 3 is a sectional view taken along line IIIIII of Fig. 2;

Fig. 4 is an end view in elevation of the thermal trip device;

Fig. 5 is an enlarged view in section along line V+V of Fig. 4; i

Fig. 6 is a detailed view of the instantaneous trip armature;

Fig. 7 is a detailed view of the movable first arrnature;

Fig. 8 is a view in detail of the trip reset button and its associated linkages; and

Fig. 9 is an exploded isometric view of the series trip coil and armatures of the overcurrent trip relay.

Referring more particularly to the drawing, Fig. 1 shows a circuit breaker 10 for protecting a circuit (not shown). Breaker 10 may be connected in series with the circuit and must therefore be made large enough to interrupt the maximum currents to be carried by such circuit. Circuit breaker 10 and its component parts are mounted on a back panel 11.

Copper bar studs 12, 16 are mounted in and project through the back panel and may be made of any known good conductive material but are preferably made of copper. Mounted on one extremity of bar studs 12, 16 are primary fingers 13 which connect breaker 10 with the circuit. Upper and lower contact blocks 14, 15 are of like construction, with the lower block 15 being mounted on back panel 11 in a position as viewed in Fig. 1 of the drawing, to the position of upper contact block 14. The bar stud 12 is firmly afiixed to upper contact block 14 by any means known in the art which will produce a low resistance joint, preferably being brazed or bolted to block 14. in like manner to a primary or trip coil 21.

A main finger 18 is fulcrumly carried by the contact block 14 to permit limited arcuate movement of main finger 18 relative to block 14. A spring 19 is positioned intermediate main finger 18 and block 14 to produce a permanent closing bias for main finger 18. A contact arm 20 is pivotally mounted on contact block 15 and mechanically and electrically engages main finger 18. Trip coil 21 is connected to contact block 15 at one of the coil ends, and the other end thereof is connected to the bar stud 16, which is connected with the primary fingers 13 mounted thereon, completing the series circuit through the breaker 10.

This current path may be designated as the primary current path which carries substantially full circuit current, which current is conducted through the breaker 10 from primary fingers 13, to bar stud 12, to contact block 14, through main finger 18 and contact arm 20, to contact block 15 and through the trip coil 21, the bar stud 16, and the primary fingers 13, thus completing the primary current path through breaker 10.-

A secondary or arcing current path includes an arcing contact arm 24 which is pivotally carried by and mounted on contact arm 20 at a pivot 25. 24 may be extruded from a good conducting material, such as copper. An arcing contact 26 is brazed to arcing contact arm 24. The arcing contact 26 being preferably made of silver tungsten alloy must be capable of Withstanding high arcing temperatures and currents. A coinplementary arcing contact 27, which extends into an arc runner 28 is similar in material to arcing contact 26, and is aifixed to and carried by contact block 14.

Arcing contact arm When breaker 10 is in a closed positiontas shown in- Fig. 1) arcing contact 26 of arcing contact arm 24 is Patented May 8, 1956' Bar stud 16 is afiixed' 1) biased by a spring 29 carried by contact arm 20, against arcing contact 27 carried by contact block 14. Thecontact arm which is integral at its top part 33 is cut apart at its bottom part 34, physically and electrically those parts are insulated from each other by an insulating washer 35. The secondary or arcing current path receives its current from contact block 14 through arcing contacts 26, 27 thence down arcing contact arm 24 through the pivot and up a leg of contact arm 21 from the lower part 34 to the upper part 33 thence down contact arm 20 to contact block 15, aiding the primary current. This secondary current aids to hold arcing contact arm 24 and contact arm 20 in a closed position because of the electromagnetic force.

The spring 19 provided intermediate the main finger and the contact block biases main finger 18 against contact arm 20 when breaker 10 is closed, just as the spring 29 exerts force on arcing contact arm 24 to bias arcing contacts 26, 27 in a closed position. An insulating disk 36 is provided intermediate spring 29 and arcing contact arm 24 to electrically insulate one from the other. Another such insulating disk 37 is provided on the arcing contact arm 24 opposing disk 36 to cushion and insulate arm 24 upon opening of contact arm 20.

The series trip coil 21 is shown as part of an overcurrent trip relay 40 which in turn is part of a breaker tripping mechanism 41 of the breaker 10. Primary or trip coil 21 is electrically connected with the circuit and is illustrated as series connected with the circuit. The trip relay 40 has a magnetizable core 42 and a trip coil 21 mounted thereon and magnetically coupled therewith.

The tripping mechanism 41 further includes a thermal trip device 45 used in combination with the relay 40. Pivotally mounted on one end of core 42 are three armatures 46, 47 and 48, all mounted on a common pivot 49 carried by core 42. A movable first armature 46 which is in the magnetic circuit of the core 42 is provided for effecting a controlling operation on occurrence of moderately abnormal circuit conditions obtained for a predetermined time. A stationary second armature 47, also in themagnetic circuit of the core, is provided as a part of thermal trip device 45. A third armature 48, mounted in like manner as the first armature, is provided for effecting a controlling operation on occurrence of instantaneous abnormally high electrical overloads. Armatures 46, 47 and 48, all of which are magnetically coupled with core 42, are more clearly seen in Fig. 9.

A secondary pickup coil 52 is carried by second armatu re 47 and is magnetically coupled therewith. The second'armature 47 is stationary, in the sense that it is merely there to provide flux in the pickup coil 52 and does not close against core 42. An air gap 53 between the second armature and the core may be adjusted by moving the second armature toward and away from the core.

A pivoted latch 54 engages and holds first armature 46 against pickup in the nontrip position. Latch 54 which may be of any known construction and configuration, is shown here as a cam plate 54 which acts upon a latch portion 56 carried by first armature 46. Latch portion 56 includes an axle bolt 57 mounted on first armature 46 and a roller 58 to hold first armature 46 in the nontrip position.

Thermal trip device 45 shown in detail in Fig. 5 further includes a shaft 60 which is mounted in and carried by breaker 10. A bimetallic element 61.is mounted adjacent shaft 60 and latch 54, having one of .its extremities firmly afiixed to shaft 60 and the other of its extremities firmly alfixed to latch 54. An electrical resistance heater 62 may .be formed to have any known configuration and is shown encompassing and positioned adjacent bimetallic element 61. Heater 62 is electrically connected with pickupcoil 52 which ismounted on the stationary second armature. Accordingly, heater 62 receives its heating current from the transformer action between pickup coil 52 and primary .trip coil 21 through air gap 53.

Means 63 shown in detail in Fig. 3 are provided for adjusting air gap 53 between second armature 47 and core 42, which means 63 include a bolt 64 affixed at one end thereof to second armature 47 and at the other end to breaker 10. A nut 65 is threadingly carried by bolt 64 to permit fine adjustment of air gap 53. A variable air gap 53 between second armature 47 and core 42 changes the amount of current in pickup coil 52 resulting from the transformer action between primary coil 21 and pickup coil 52. This, of course, changes the heating characteristics of heater 62. Adjustment of air gap 53, therefore, permits one to interchange primary coils .of various current ratings in the relay 40, while maintaining a proper relationship between primary and pickup coils to effect the desired time delay by adjustment of air gap 53.

The heating action, transmitted by convection and radiation from heater 62 to bimetallic element 61, causes the spiraled element 61 to exert a turning torque and move the cam plate 54 through an arc sufiicient to release latch portion 56 freeing first armature 46 for closing. As first armature 46 closes it actuates a means for tripping the breaker, shown as tripping mechanism 41 in Fig. 1.

Means are also provided for adjusting thearcuate travel of the latch 54 with respect to roller 58 of latch portion 56, which is shown in detail in Figs. 4 and 5, and includes a worm gear 68 mounted on shaft 60 and doweled thereto by a pin 69 through the hub of the gear 68. A spacer 74 is mounted on shaft 69 intermediate worm gear '68 and latch 54. Adjustment of latch 54 with respect to latch portion 56 is elfected by selective rotation of a knob 79 to rotate a worm 71 which in turn causes rotation of worm gear 63. Rotation of worm gear 68 turns shaft 60. Shaft rotation is transmitted through bimetallic element 61 to latch '54 to selectively position the same with respect to roller 58 of latch portion 56. In this manner it is possible to adjust the position of latch 54 on roller 58 of the movable armature which in turn determines the amount .and the duration of the overload current necessary to release first armature 46 for closing.

Movable first armature 46 (Fig. 7) and instantaneous trip third armature 48 (Fig. 6) are shown in detail to illustrate their structures and show the adjustable trip hammers 72, 73 carried, respectively, by first and third armatures 46, 48.

The first and third armatures 46, 48 may be adjusted for various pickup ratings desired by a pickup adjusting structure 75 shown in detail in Figs. 2, 3 and 9. Each of armatures 46, 48 is biased by a spring '76, against pickup, one end of which is fastened to the armature and the other end of which is carried by a bolt 77. Bolts 77 extend through plate 78 which is carried by breaker 10, .and they are utilized to adjust the spring tension of springs 76 for increasing or decreasing the amount of flux needed to effect pickup of armatures 46, 48. These pickup adjustments are usually made in the factory for predetermined trip coil ratings.

Either one of the tripping hammers 72, 73 may initiate when the block 91 is rotated. A Y-shaped star wheel 93 is pivotally carried by breaker 10. A roller 94 is mounted on .one of the legs of the star wheel 93 and is in engagement with block 91. A trip rocker 97, having first and second rocker arms 98, 99 is pivotally carried by breaker 10 at a pivot 100. Pivotally mounted on first rocker arm 93 is a roller 102 which engages one of the legs of the star wheel 93 in endwise engagement. The second rocker arm 99 has an outer surface 103 eccentrically ground with its geometric center within the rocker 97 but above pivot 100. Therefore pressure exerted on the outer surface 103 of the second rocker arm 99 will produce a clockwise turning torque (as viewed in Fig. '1') on the rocker 97 about its pivot 100. A cam plate 104 having an arcuate inner cam opening 105 is mounted on breaker 10 and is adjacent rocker 97. An operating arm 106 having a first end 107 and a second end 108, has its first end 107 mounted adjacent cam plate 104. The first end also carries a pin 111 which projects through the cam plate 104 at the inner cam opening 105. The pin 111 has a roller 112 journaled thereon which roller is free to rotate but is firmly held against axial movement of pin 111 and operating arm 106. The roller 112 abuts and engages the outer surface 103 of the second rocker arm 99. A trip latch 113 having a cam surface 121, is pivotally carried by the operating arm 106 at its pin 111. The second end 108 mounts a pivot 115 for pivotally attaching an insulating arm 116 and a first indicator link 119.

Operating arm 106 has a changing pivot point which shifts from a center point 120 intermediate the ends 107, 108 when the contacts are opened to a point coinciding with the pin 111 of the first end 107 when the contacts are reclosed. A trip pin 114 is mounted on a trip latch bar 122 and engages cam surface 121 of trip latch 113. Trip latch bar 122 is pivotally mounted on a shaft 123. A cam rider 126 is carried by trip latch bar 122. A spring 127, biasing the trip latch bar 122 and its trip pin 114 against cam surface 121 of trip latch 113, is connected at one end thereof to breaker 10 and the other end thereof connected to the operating arm 106 intermediate the center point 120 and the second end 108.

A manual reclosing mechanism 130 comprises a handle 131 mounted on a shaft 132 journaled in breaker 10. a

A closing cam 133 is aifixed to shaft 132 and rotates therewith upon rotation of handle 131. Handle 131 is spring biased by a torsion spring 134 which returns the handle 131 to a normal reset position upon release. Cam rider 126 slidingly engages closing cam 133 when the breaker has tripped and the contacts are open. Reclosing is effected by rotating handle 131 whereupon the cam rider 126 follows closing cam 133 and counteracts the bias of the spring 134 to reset the trip pin 114 against cam surface 121, thereby resetting trip latch bar 122.

Insulated arm 116, which is pivotally attached to the second end 108 of operating arm 106, is also pivotally attached at its opposite extremity to contact arm 20 at pivot 135. First indicator link 119 is pivoted at pivot 137 and has a second indicator link 138 pivotally attached at pivot 139. Thesecond indicator link 138 is attached to an open-closed breaker indicator 140 pivotally mounted on pivot 141 carried by breaker 10.

A phase trip indicator 145, shown in Fig. 2 and dotted in Fig. 3, is afiixed to an indicator trip shaft 146 journaled by breaker 10. Phase trip indicator has a latch portion 147 which engages a phase indicator trip pin 148. The phase indicator trip pin 148 is mounted on a phase indicator trip bar 149. As the armature '46 closes it engages the phase indicator trip pin 148 and disengages the phase indicator trip pin 148 from latch portion 147 freeing phase trip indicator 145. Phase trip indicator 145 is caused to rotate clockwise by its own weight and is aided by a torsion spring 150 mounted on indicator trip shaft 146. The clockwise rotation of phase trip indicator 145 is also aided by spring 151 (shown in Fig. 2) which also exerts a positive turning action on phase trip indicator 145. A phase trip indicator reset button 155 (Figs. 1 and 8) is mounted in breaker 10 for indicating the tripped phase by popping out on tripping and may be pushed in toward the breaker for resetting the indicator 145 by rotating shaft 146. Manual pressure is applied to button 155 and is translated into rotary motion exerted on shaft 146 by links 156, 157 and 158.

The opening of breaker 10 (shown in Fig. 1 in a closed position) which operation is hereafter explained, occurs when overload currents cause the circuit it is protecting to be interrupted. The operation of only first armature 46 will be discussed. However, it should be clear to those skilled in the art, that any high instantaneous overload 6 current would cause third armature 48 to pick up and hammer 73 will initiate the trip mechanism 41.

As a prolonged overload current condition is presented to breaker 10, trip coil 21 presents a magnetic flux to relay 40. This magnetic flux attracts first armature 46 for pickup. However, armature 46 is restrained from pickup by spring 76. But when the overload current through trip coil 21 is great enough, armature 46 will overcome the tension of spring 76 and pick up. Armature 46, however, is further restrained by latch 54 and as trip coil 21 presents an increased magnetic flux, transformer action between coil 21 and pickup coil 52 induces an electromotive force in pickup coil 52, depending on the air gap 53. Accordingly, a current flows through pickup coil 52 and to heater 62. As this current increases the heating effect of heater 62 causes bimetallic element 61 to twist spirally. This twisting causes rotation of latch 54 on shaft 60 and with respect to roller 58. As rotation continues latch 54 rotates free of roller 58 freeing first armature 46 and permitting it to close.

As first armature 46 approaches closing, it engages the phase indicator pin 148 causing it to be dislodged from latch portion 147, thereby dropping phase trip indicator 145 into tripped indicating position.

As pickup of first armature 46 is effected, it also causes tripping hammer 72 to engage block 91 and cause block 91 to be rotated counterclockwise a portion of a revolution. The counterclockwise rotation of block 91 causes roller 94 of one of the legs of star wheel 93 to transmit a clockwise rotation to the star wheel 93. As the star wheel 93 rotates clockwise, roller 102 mounted on first rocker arm 98, which roller 102 endwise engages one of the legs of star wheel 93, drops from endwise engagement therewith freeing rocker arm 97 for clockwise rotation about its pivot 100. Roller 112 of operating arm 106 exerts the forces of spring 127 and breaker contact springs 19, 29 against outer surface 103 of rocker arm 97 and further causes a positive clockwise rotation of rocker arm 97. Roller 112 of operating arm 106 becomes disengaged from outer surface 103, thus permitting pin 111 carried by the first end 107 of operating arm 106 to follow the cam opening 105 in cam plate 104. This movement of first end 107 of operating arm 106 permits operating arm 106 to pivot about the center point 120. Trip latch 113 has its latched cam surface 121 released from the trip pin 114, also freeing trip latch bar 122. The trip latch bar 122 rotates clockwise about its pivot 123, being limited by a stop 160. This places cam rider 126 in engagement with closing cam 133. The pivoting of operating arm 106, aided by the spring 127 and breaker contact springs 19, 29, causes second end 108 to move counterclockwise to aid insulated arm 116, which being attached also to the contact arm 20, draws the contact arm 20 out of contact with the main finger 18. Breaking the contact of contact arm 20 and main finger 18 transfers the current flow to arcing contacts 26, 27 which open after main finger 18 and contact arm 20. The are drawn in the opening of arcing contacts 26, 27 travels upwards toward an arc chute 161 defined by notched barrier plates 162 and leaves the arcing contacts 26, 27 to advance along arc runner 28 and an arcing jet 163 carried by arcing contact arm 24. Arc chute 161, barrier plates 162 and the addition of a ferromagnetic core 164 adjacent the underside of arc runner 28, in combination with the thermal and magnetic conditions caused by the arc, all aid in its extinguishment.

At the same time insulating arm 116 is drawn back to open breaker 10, the first indicator link 119 pivots clockwise about pivot 137 which causes link 119 to act on second indicator link 138 to pivot the open-closed breaker indicator 140 about pivot 141 indicating the open condition of the contacts of breaker 10.

After a reasonable time has elapsed to permit cooling of bimetallic element 61, a clockwise rotation of handle 131 of manual reclosing mechanism 130, after pushing the phase trip indicator reset button 155, will restore breaker to a closed position.

The particular embodiment of the invention illustrated and described herein is illustrative only and the invent-ion includes such other modifications and equivalents ,as may readily appear to those skilled in the art, Within the scope .of theappended claims.

It is claimed and desired to secure by Letters Patent:

1. .In an alternating current circuit breaker, the combination comprising a core, a primary coil electrically connected with said circuit breaker and inductively coupled with said core, a movable first armature in the magnetic circuit of said core, a stationary second armature in the magnetic circuit of said core, a secondary pickup coil inductively coupled with said second armature, a thermal trip device including a shaft, a pivoted latch, said latch engaging and holding said first armature against pickup in its nontrip position, a bimetallic element mounted adjacent said shaft and said latch having one of its extremities firmly aifixed to said shaft and the other of its extremities .firmly afiixed 'to said latch, a heater adjacent said bimetallic element, said heater being electrically connected with said pickup coil, 'means for adjusting an air gap between said second armature and said core, and means actuated by said first armature for tripping said circuit breaker.

2. "In an alternating current circuit breaker, the combinationcomprising a core, a primary coil electrically connected with said :circuit breaker and inductively coupled with said core, .a movable first armature in the magnetic circuit of said core, a stationary second armature in the magnetic circuit of said core, a secondary pickup coil indntively coupled with said second armature, a thermal trip device including .a shaft carried by said breaker, a pivoted latch journaled on said shaft and free to rotate with respect thereto, said latch engaging and holding said first armature against pickup in its nontrip position, a bimetallic element mounted adjacent said shaft and said latch having one of its extremities firmly afiixed to said shaft and :the other of its extremities firmly affixed to said latch, a heater adjacent said bimetallic element, said heater being electrically connected with said pickup coil, means for adjusting an air gap between said second armature :andsaid core, and means actuated by said first armature fortrippingsaid circuit breaker.

3. In :an alternating current circuit breaker, the combination comprising a core, a primary coil electrically connected with said circuit breaker and inductively :coupled with said core, :a movable first armature in the magnetic circuit of said core, a stationary second armature in the magnetic circuit of said core, a secondary pickup coil inductively coupled with saiclsecond armature, a thermal trip device including a shaft, .21 pivoted latch, :said latch engaging and holding said first armature against pickup of its nontrip position, a bimetallicelement mountedadjacent said shaft and said latch having one of its extremities firmly affixed to said shaft and the other of its extremities firmly-affixed to said latch, a heater adjacent and encompassing said bimetallic element, said heater being electrically connected with said pickup coil, means for :adjusting an air gap between said second armature and said core, and means actuated by said first armature for tripping said circuit breaker.

4. In an alternating current circuit breaker, the combination comprisinga core, a primary coil electrically connected with said circuit breaker and inductively coupled with said core, a movable first armature in the magnetic circuit of said core, a stationary second armature in the magnetic circuit of said core, a secondary pickup coil inductively coupled with said second armature, a thermal trip device including a shaft, a pivoted latch, said latch engaging and holding said first armature against pickup in its nontrip position, a bimetallic element mounted radjacent said shaft andsaid latch having one of its extremities firmly afiixed to said shaft and the other of its extremities firmly affixed to said latch, means for adjusting the arcuate travel of said latch with respect to said first armature, a heater adjacent said bimetallic element, said heater being electrically connected with said pickup coil, means for adjusting :an air gap between said second armature, and said core, and means actuated by said first armature for tripping said circuit breaker.

5. In an alternating current circuit breaker, the combination comprising .a core, a primary coil serially con nected with said circuit breaker and inductively coupled with said core, a .movable first armature carried by and in the magnetic circuit of said core, a stationary second armature carried by and in the magnetic circuit of said core, a secondary pickup coil inductively coupled with said second armature, a thermal trip device including a shaft, a pivoted latch, said latchengaging and holding said first armature against pickup in its nontrip position, a bi metallic element mounted adjacent said shaft and said latch having one of its extremities firmly afiixed to said shaft and the other of its extremities firmly aflixed to said latch, aheater adjacent said bimetallic element, said heater being electrically connected with said pickup coil, means for adjusting an air gap between said second armature-and said core, and means actuated by said first armature for tripping said circuit breaker.

6. In an alternating current circuit breaker, the combination comprising a core, a primary coil serially connected with said circuit breaker and inductively coupled with said core, a movable first armature in the magnetic circuit of said core, a stationary second armature in the magnetic circuit of said core, a movable third armature in the magnetic circuit of said core, said first, second and third armatures being mounted on a common pivot and carried by the common core, a secondary pickup coil inductively coupled with said second armature, a thermal trip device including a shaft, a pivoted latch, said latch engaging and holding said first armature against pickup in its nontrip position, a bimetallic element mounted adjacent said shaft and said latch having one of its extremities firmly affixed to said shaft and the other of its extremities firmly aflixed to said latch, a heater adjacent said bimetallic element, said heater being electrically connected with said pickup coil, means for adjusting an air gap between said second armature and said core, and

means actuated by said first armature for tripping said circuit breaker.

References Cited in the file of this patent UNITED STATES PATENTS 1,820,877 Wood Aug. 25, 1931 2,024,217 Grady Dec. 17, 1935 FGREIGN PATENTS 257,594 Great Britain Mar. 17, 1927 

